When an electric motor is started, the electric current drawn by the motor can be six times the steady state current once it reaches full speed. Manufacturing equipment and assembly lines often have a number of relatively large three-phase electric motors which start simultaneously thereby placing very large current demands on the electrical distribution system feeding the equipment or assembly line.
In order to reduce this start-up current consumption, large alternating current electric motors are often operated by a controller, as is schematically shown in FIG. 1. The motor controller 10, such as an Allen-Bradley Company model SMC-150, controls the application of electric current to the motor 12 by means of a thyristor switch module 14. The switch module includes three pairs of silicon controlled rectifiers (SCR's) 16, 17, 18, each of which couples one of the three alternating current (A.C.) supply lines A, B, or C to one of the motor stator windings 21, 22, 23. The SCR's in each pair are inverse parallel connected to provide a bidirectional electrical switch. Three trigger outputs from the motor controller are coupled by separate isolation transformers 20 to one of the SCR pairs 16, 17 or 18. The trigger output is coupled to the transformer primary coil and the gate of each SCR is connected to a separate one of two secondary coils to provide isolation. Alternatively, a single triac could be used in place of each SCR pair.
When the motor 12 is to be started, the equipment operator applies a starting signal to the motor controller 10. As is well-known, the motor controller 10 gradually increases the amount of current applied to the motor by regulating the duty cycles of the SCR's. In doing so, the controller turns on the SCR's initially for only a brief portion of each half-cycle of the A.C. voltage for the corresponding electricity phase. The controller then gradually increases the half-cycle on time of the SCR's until they are constantly turned on at which time the motor is at substantially full speed. This technique reduces the current consumption and torque of the motor during start-up as compared to a hard switching of the full supply line voltage across the motor.
These motor controllers often did not provide a mechanism for braking the motor when it was stopped. In response to an operator input to stop the motor, the basic controller simply turns off the SCR's allowing the motor to coast to a stop, slowed only by friction. If the motor is coupled to a mechanical load with considerable inertia, the motor and the load will continue to move for some time after the power is shut off. In many industrial applications of motors, it is important for convenience and efficient use of the driven equipment to stop this continued movement as fast as possible. Merely allowing the motor to coast to a stop is unsatisfactory. Heretofore, a mechanical brake was often coupled to the equipment and engaged when the power was turned off.
As an alternative, a direct current was sometimes applied to the stator windings of an alternating current motor to provide a braking action. In order to electrically brake an alternating current motor, it is necessary to generate a torque in the direction opposite to the direction of the rotation of the rotor. In the direct current injection method of the prior art, the torque is produced by the rotor attempting to rotate in the presence of a steady magnetic field produced by the direct current applied to the stator winding. The rotating direction of the rotor's magnetization leads the direction of the magnetic field produced by the direct current through stator winding. The tendency of the rotor magnetization to align itself with the stator's magnetic field creates an alignment torque which produces a braking effect on the rotor. As is well-known, this torque is equal to the product of the stator magnetic field strength and the rotor magnetization together with the sine of the angle between the two.
Another method of braking involves reversing the A.C. phases applied to the motor. This latter technique requires a mechanism to detect when the motor stops and then to shut off the electric current so that the motor does not begin running in the reverse direction.